The thermal resistance of a fabric or composite is measured in accordance with standardized test methods, both NFPA and ASTM.
The test specimen, mounted in a sample holder, is positioned above the heat source and is protected by a water-cooled shutter before and after the test run. The heat exposure is generated through a combination of a convective heat source provided by two laboratory burners and a radiant source provided by a bank of quartz tubes. The instrument is calibrated to ensure the delivery of an exposure averaging 2.0 cal/cm2 sec (8.4 watts/cm2).
The heat transferred through the test specimen is measured by a sensor. An especially designed data acquisition software program accurately records temperature measurements from the sensor. Rate of rise in temperature, or the slope of the temperature vs. time trace is used in conjunction with calorimeter constants to compute heat flux. A square wave exposure sequence is used so that results can be related to values obtained by Stoll (1). A human tissue tolerance overlay obtained by integration of the Stoll curve with respect to time is used to determine tolerance times to second degree burns. A human tissue tolerance overlay is used to determine tolerance times to second degree burns. The TPP values are the product of the incident heat flux and the recorded tolerance time to second degree burn (cal/cm2).
1. Stoll, A.M. and Chianta, M.A., Method and Rating System for Evaluation of Thermal Protection. Aerospace Med. 40: 1232 – 1238, 1968